Robust and simple to configure cable-replacement system

ABSTRACT

In a modular signal mirroring system each point-to-point RF transceiver end has a controller module coupled to one or more I/O modules. The I/O modules have various input and output circuits. A signal received at the near end is reconstructed at the far end after being transmitted in an RF packet. The reconstructed signal may be the same as the input signal, inverted from the input signal or level-shifted from the input signal. It is representative of the input signal following the input signal&#39;s state after a time-quantization latency. Transmission between the two ends is via a periodic transmission controlled in a master slave protocol. If a transmission is not received in a timely manner or, in some systems, if intentional interference with transmissions is detected, the reconstructed signal is forced to a safe state determined by local switch settings. The settings include the last known good state.

BACKGROUND OF INVENTION

1. Field

This disclosure relates to transmitting and receiving the state ofelectrical signals via a point-to-point radio frequency link; morespecifically to radio-based cable replacement systems.

2. Background Art

Industrial facilities often have sensors and controllers that are remotefrom a central monitoring and control station. This can be in powerplants, petroleum, and chemical operations as well as many others.Typically, long electrical cables convey the signals between remotelocations and a control room. There are now many devices known thatreduce the amount and length of cabling by using a network, particularlya radio frequency based network, to convey signals. In these systems adevice at one end receives several electrical inputs, determines theirstates and transmits the state information to a distant unit. Thedistant unit receives the data, and based on it, sets its severaloutputs to correspond to the state of the first unit's inputs, therebyacting as a cable replacement. Signals can be from the field to acontrol room, from a control room to a remote location, or otherwise ata distance from each other. Many of these systems are susceptible toissues and disadvantages including complexity of configuration,unpredictable latencies, single points of failure, and difficulty indiagnosing problems.

Some of those issues and disadvantages are radio interference, failurein the firmware or hardware of the end-point devices, network failure,and loss of power to the devices. Inevitably some degree of increasedlatency is also introduced.

Other disadvantages can include ease of configuration. While running along cable can be challenging in some locations, there is noconfiguration involved other than determining which conductor at one endcorresponds to which conductor at the other end. In contrast,radio-frequency network-based cable replacement systems usually requiredownloading software from the manufacturer's web site, using a computerin the field to download settings to each unit, and many more steps.While growing in use, these systems can benefit from simplerconfiguration and more robust and diagnosable radio linkages.

BRIEF SUMMARY OF THE INVENTION

One end of a transmitter/receiver pair in a point-to-point radiofrequency connection can characterize the state of an electrical inputsignal and transmit a block of information including a field of datarepresentative of that state. The other, receiving, end can receive thatblock of information and can detect if the block of informationrepresents a valid and un-interfered with transmission. It can thenrecreate the state of the original input signal conveyed in the datafield on a mirrored local output circuit. Alternatively, the receivingend can set the output circuit to a predetermined state mapped from thedata field, possibly inverting the signal or translating it to analternate signaling scheme.

In cases where the received transmission or data within a transmissionis determined to be invalid, corrupt, un-timely in arriving, jammed,etc., the receiver can cause its outputs to be forced to a predetermineddefault “fail-safe” state. This state can be separately settable foreach output and each output's default state can be determined bysettings of physical switches used as inputs to signify choices amongpredetermined rules.

In some embodiments, the paired end units can be transceivers with bothends having inputs and outputs, providing a bidirectional operation.Although the end units making up the pair can be very similar, thesystem can be configured in a master/slave arrangement with eachrespective unit operating according to a distinct programming. Amongother ways of pairing transceivers they can be automatically pairedbefore being shipped. This includes loading the mate's radio address andcryptographic keys in the units.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become better understood when the followingdetailed description is read with reference to the accompanying drawingsin which like reference designators represent like parts throughout thedrawings, wherein:

FIG. 1 shows a simplified block diagram of a first embodiment of amaster/slave, paired, mirrored I/O, wireless cable replacement system;

FIG. 2 shows a physical view of the paired system of FIG. 1;

FIG. 3 shows a simplified block diagram of the controller/radio moduleshown in FIG. 1;

FIG. 4 shows a simplified block diagram of an I/O module that iscompatible with the system of FIG. 1;

FIG. 5 shows a simplified view of the timing of packets exchangedbetween the ends of the paired system of FIG. 2;

FIGS. 6A-6B show a hypothetical timing diagram of the local input andremote output signals seen in FIG. 1;

FIG. 7 is a flowchart of the actions of a master controller in thewireless cable replacement system of FIG. 1;

FIG. 8 is a flowchart of the actions of a slave controller in thewireless cable replacement system of FIG. 1;

FIG. 9 is a flow chart of a handling of exception conditions for bothmaster and slave operations shown respectively in FIG. 7 and FIG. 8;

FIGS. 10A-10B are flowcharts of the actions of the bi-directional I/Omodule of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

Structure

Reference numerals are used to designate portions and aspects of thesystem. The same portion or aspect used in various positions andcontexts will retain the same reference number. Due to the manysymmetric aspects of the end points there are many cases of an instanceof a system portion that is duplicated but operates in a distinct mode.In those cases the numeral has a prime mark.

FIG. 1 shows a simplified block diagram of an examplewire-to-wireless-to-wire system. In this simplified example for clarity,only two I/O modules are associated with each end-point. Also, only twoelectrical signals are shown with each I/O module. The master side 100has a radio module 101 coupled to a controller module 102 that arecommonly packaged 500. The controller has a UART 103 that is used as alocal communication channel to multiple I/O modules 120, 140.

One I/O module 120 has one input labeled 121 and one output labeled125M. During operation, the system acts to reflect, or mirror, the stateof input 121 to the output 121M in the slave system 200 and also toreflect the state of slave side input signal 125 to the master sideoutput 125M. Dashed lines 121V, 125V illustrate the virtual transfer ofthese signals from end to end.

The slave end 200 has a radio module of the same type as the master'scoupled to a controller 101′. The controller is physically the same typeof unit as the master-side controller but programmed or configured tocarry out the role of slave. Similar to the master side, the radio andcontroller are commonly packaged and the controller communicates over amulti-drop, sub-system, inter-connect RS-485 bus 104′ to connected I/Omodules. The first I/O module on the slave 120 side is the same type asthe first module 121 on the master side. It has one input and oneoutput. The second module 150 is not the same type as the master side'ssecond module 140, however they are complimentary. The master side's twoinputs 141 and 142 are reflected to the slave side's two outputs 141Mand 142M. Specifically, the second I/O module 140 on the master side hasonly inputs; they are labeled 141 and 142. The operation of the systemresults in their states being reflected to the slave side's outputslabeled 141M and 142M. Dashed lines 141V and 142V illustrate the virtualtransfer of these signals from master end to slave end.

The physical packaging of these modules is shown in FIG. 2. The modulesare supported by mechanical connection to a DIN 15 rail. The rail has apassive backplane 202 to carry the RS-485 bus among the modules. Theleft side of each subsystem has a module 500, 500′ containing thecontroller and radio subsystems. The other two modules are I/O modules.As shown, they are a four-input/four-output module and an eight-inputmodule. Terminal blocks 201 for the electrical connections are on thetop and bottom of the I/O modules and an antenna 200 can be local to thecontroller module.

The next two figures show block diagrams of particular modules in moredetail. FIG. 3 is a block diagram of the controller and radio module.The radio module 101 in this case is a Digi International XBee-Prospread spectrum version operating in the scientific and industrial 900MHz band. The operation of the controller is firmware embedded in aMSP430 microcontroller 102. The microcontroller connects to the Digiradio via a serial port 110 through a radio module port 501. The secondserial port of the MSP430 is used for the multi-drop, sub-system,inter-connect bus 104 after being level-translated by circuitry 105 toRS-485 signal. The unit also has a push button 111, a USB port 112, andseveral indicators.

The I/O module seen in the block diagram FIG. 4 is afour-input/four-output unit 120 (only one input and one output of whichare shown in the more simplified FIG. 1). This particular module is alsocontrolled by firmware embedded in a TI MSP430 microcontroller 130 thatis programmed and configured with firmware to carry out the actions ofthe I/O module. Four input signals 121, 122, 123, and 124, are receivedby signal conditioning and receiving circuitry 134 under the control ofthe programming of the microcontroller. This received data is madeavailable over the multi-drop, sub-system, inter-connect bus 104 usingthe modules' protocol for mutual communication. Data sent to the moduleover the multi-drop bus is provided to output latching and signalconditioning circuitry 135 to be provided to output circuits 125M, 126M,127M, and 128M.

In this example, a rotary switch 131 is used to determine a moduleaddress to uniquely identify each I/O module. DIP-switches 132 are usedto indicate a desired output in a default or fail-safe condition. In thecurrently presented embodiment each output may be indicated to be set inone of three states upon a fail-safe state. One is “high”, one is “low”and the other is the last known good transmitted state. This choice ismade by a user via the setting of the appropriate DIP-switches.

Those knowledgeable in the field will understand that low and high canbe taken to designate logical states of a digital signal and do notnecessarily correspond to the actual magnitude of a voltage or currentbeing higher or lower. In other modules analog signals maybe supportedand a different designation of the fail-safe electrical conditions maybe required, for example, a particular voltage level or impedance. Amulti-valued state could also be supported.

Operation

There are various phases of the operation that are generally common tothe master and to the slave. They include installation, initialization,retrieving the state of the input signals to the I/O modules,transmitting the state of those input signals, receiving informationabout the other side's input signals, and sending that information tothe appropriate I/O module for outputting. There are also variouserror-checking tasks performed.

Installation

Due to the packaging of the presently described embodiment, theinstallation of modules is simply performed by attaching them to a DIN15 rail that has a passive RS 485 backplane 202. One controller moduleand up to sixteen I/O modules can be installed on the bus to create oneend of a paired system. To complete the installation each I/O module'saddressing rotary switch is set to a unique value, any fail-safe statechoices are made and encoded in DIP-switches; and desired signalingwires are attached to terminal blocks.

Calling the first installed system the near end, these steps arerepeated at the far end. One of the two ends is designated as a masterand the other as a slave. That does not convey a particular sense of“importance” of signal direction or of designated location. It is merelya characteristic of the intra-unit communication protocol chosen in thisembodiment.

Controllers at each end are paired units for mutual radioaddressability. Also the I/O modules are compatible, interworkable unitsat corresponding rotary switch addresses. That is, the I/O module at thenear end with an address of 1 will exchange information with the I/Omodule set to address 1 at the far end and therefore must be compatiblein order to provide a useful function. The radio modules used in thisembodiment are Digi International XBee modules designed to operate usingthe IEEE 802.15.4 standard protocol. This standard is intended forso-called low-rate data transmissions.

In the case of a symmetric I/O module, as in the unit of FIG. 4, thenear and far modules can be identical units. Another configurationoption is to have units that both are described by FIG. 4 at a blockdiagram level but might have different signal levels. One proximate to acontrol station might be TTL logic levels while its, otherwise similarmate, might have opto-isolated current-based I/O levels. In that case asignal would track its corresponding signal but would not be strictlymirrored. Another example would be an inverting mirroring.

Another case of mated modules might be a far end module at address 2with eight inputs and a mated near end module at address 2 with eightoutputs. These would not be identical unit types, but they would becompatible units.

Initialization

On power on or hard reset, the system at each end will poll for I/Ounits on its half-duplex, multi-drop bus at addresses from 0 to 15. Inthis example embodiment the I/O modules respond in fixed time slots withfixed size packets. The time slots are initially determined by therotary switch settings, higher addresses having a time slot after loweraddresses. The controller can perform some system checks at this time tolook for address conflicts as well as to make an internal map of theinstalled module types. The controller can also reassign moduleaddresses for improved efficiency. These operations are doneindependently at both the near and far end.

General Continuous Operation

After the initializations, inter-system communication can proceed. Themaster directs a periodic burst of a transmission to the unique radioaddress of the slave in a unicast manner. In one mode this can be onceper second. The burst will contain a header and a fixed size packet foreach I/O module found installed by the master. These packets werepreviously retrieved by the controller from each respective I/O moduleover the multi-drop bus.

The slave, that has been waiting quietly for a transmission from themaster, receives the periodic burst and breaks the received data intothe header and a per I/O module fixed size packet. The local multi dropbus is used to send those packets to their respective modules. Based ontheir addresses the I/O modules receive those packets and use theinformation to mirror the master-end reflected signals.

To finish the symmetry of the mirrored system, the slave controllerpolls its I/O modules for their respective inputs, creates a compositedata packet, and transmits it, addressed in a unicast fashion, to themaster. As long as this is done before the master's next periodictransmission it should be readily received by the master with noconflict, flow control requirement, or other complex protocolrequirements. The master receives this data and sends respective packetsto its I/O modules.

FIG. 5 shows a simplified view of the transmissions. At one secondintervals the master emits sequence-numbered data packets 300M, 301M.After receiving and processing each of these packets the slave endacquires its local I/O modules' data and responds by emitting acorresponding packet 300S, 301S. A time-quantized mirroring, as seen inFIGS. 6A and 6B, is a result of the periodic sampling and transmittingof the electrical input signals' states.

Using the signals of module 120 and 120′ of FIG. 1 as an example, FIGS.6A and 6B show the periodic sampling of local signals being turned intodistant mirrored signals. FIG. 6A shows signals at their respectiveorigination points and FIG. 6B shows the timing of the mirrored versionsof those signals. Time marks represent seconds.

A transition labeled 321 engenders transition 322, transition 323engenders transition 324, and transition 325 engenders transition 326.One thing to note is that a signal that changes more rapidly than thesample time can have a “transient” transition 327 that has no effect atthe far end.

Master Operation

The flowchart of FIG. 7 shows a simplified view of the master's actions.In step S100 the controller polls its locally connected I/O modules. Itreceives a fixed size data packet from each installed module.

In step S101 the packets from the various I/O modules are compiled intoa full packet for transmission including a header with addressinginformation, a sequence number and a map of the state of that end of thesystem. In the present example the data is encrypted with keys that areconfigured into the controllers at the time of manufacturing.

The controller then sends this assembled packet to the radio module viaa serial bus. The radio module then sends the packet over the air S102.The radio is one of many radio module types made by Digi International.Digi offers a variety of radio modules differing in RF frequencies andtransmission types, but having a common form-factor and system sideinterface. This allows variations of the present embodiment withinterchangeable radio types. Options include frequency hopping, spreadspectrum, etc. Of course, both ends of a point-to-point system will havemutually inter-workable radios.

After transmitting, the master end is available to receive S103, thecorresponding response from its associated slave. During this time, atime-out period is calculated S104. If no proper response is receivedafter a predetermined time, then control is sent to a fail-safe sequenceshown in FIG. 9.

When a proper and timely packet is received from the slave, it is brokenup into sub-packets, each sent S105 to a respective I/O module over thelocal multi-drop bus. The header packet can also be checked for propersequence number and other configuration compatibility.

As the master, this sequence of actions determines the periodicity ofsystem-wide transmission. In this embodiment there are two rates oftransmission. As mentioned above, one of the options is once per second.This option can be very valuable for slowly changing signals. Batterylife and airtime congestion are both conserved. However if signals arechanging more rapidly, or if reduced latency is desired, the unit can beset in a “fast” mode. The mode is toggled by the push button 111 shownin FIGS. 1, 2, and 3. In the fast mode the repetition rate depends uponthe number of I/O modules installed. With only one module the repetitionrate is every 100 milliseconds. As more modules are added the “fast”rate approaches the slow rate's one-second value.

A determination is made S106 as to the unit being in a fast or a slowrepetition rate state. Next, an appropriate delay S107, S108 isinserted. After the delay, the sequence is re-entered.

Slave Operation

FIG. 8 is a flowchart of the operation of the controller at the slaveend. Its operation is complementary with that of the master to achievethe system-wide results.

In an initial step the slave listens for a good unicast packet addressedto it from its paired master S200. That process continues S201 until atime-out occurs or a good packet is received. Upon receiving a good andtimely packet, it is broken into sub-packets and delivered to therespective I/O modules S202 over the local multi-drop bus foroutputting.

Next, the I/O modules are polled in turn by the controller to get theirrespective input data and assemble into a packet for transmission S203.The controller sends that packet data to the radio module. The radiothen transmits S204 the packet over the air addressed to the pairedmaster. After a transmission, the slave returns to the waiting step.

Time-Out and Tampering Operation

The detection of an interruption in a sequence or series of transmittedpackets or a break in valid transmissions is not always black and whitebut can involve heuristics. A packet that arrives earlier or later thanexpected, a packet with an out-of-order sequence number or a change insignal strength can all contribute to a suspicion of tampering,interference, or technical failure. Although not always correctly, logicin the controller can conclude that a third party tampering or jammingattempt is occurring, a technical failure has occurred, or that normaloperations are proceeding.

A fail-safe or default condition can be initiated by these decisionsoccurring in the controller or possibly in individual I/O modules. Insome embodiments it may be possible and valuable to attempt todistinguish between “innocent” failures and various types of third partyattacks and for an embodiment to take differing actions under differingcircumstances.

The flowcharts of FIGS. 7 and 8 show an exiting path in the case of atime-out. FIG. 9 is a very simplified view of the flow of actions fromthat point and shows the response to a time-out event. It also showsoptional steps in the case of a tampering detection. A tamperingdetection could be assumed if there are excessive over-the-aircollisions, possibly indicative of a jamming denial of service attack.It might be assumed based on out-of-sequence packets that might be froma playback attack. Some embodiments may also have detection of someforms of physical tampering. Tampering suspicion is a second flow shownin FIG. 9.

A time-out flow from either FIG. 7 or 8 is directed to step S300 in FIG.9 and a tampering detection (not shown in the other flowcharts) wouldlead to step S301, also seen in FIG. 9.

Time-out and many tampering determinations would be made by logicoperating in the controller. These determinations need to becommunicated to the various I/O modules to direct them to takeappropriate action. Header information in packets directed to each I/Omodule will indicate a time-out occurrence in step S300 or a suspectedtampering in step S301. Each module can take action, or not, on thisinformation.

Common to both paths, in step S302 any questionable packets arediscarded and then operations are resumed.

Additional Robustness Feature

One category of attack or error that can interfere with operationinvolves a radio module getting into an unresponsive state. Logic in thecontroller portion can detect this unresponsiveness and control a signalto perform a hard reset of the radio module. Alternately, the controllerportion could control power to the radio module and accomplish a fullre-initialization by power cycling the radio.

I/O Module Operation

FIGS. 10A and 10B show flowcharts of the high-level operation of an I/Omodule like the one of FIG. 4. When polled for its received input, themodule senses the state of its inputs S400, assembles a packetrepresenting that data S401, and sends a packet to the controller S402over the local multi-drop bus.

Separately, when an I/O module receives the packet over the multi-dropbus it then determines if it is a good packet S403 as seen in FIG. 10B.The packet may contain a time-out code or a tampering code from thecontroller. Also, the I/O module may have its own end-to-end tamperingor problem detection between it and its other-end mate.

If it is a good packet, in step S405 it sets the output circuit to theelectrical states dictated by the packet's data. Optionally it alsostores this as a last-known good packet S404. In I/O modules with afail-safe feature, a time-out or a tampering detection can cause the I/Omodule to set its various outputs to a fail-safe state based onsettings. In that case, in step S406 the DIP-switches are read and atermination is made to either set each output to a preset electricalstate, or to set it to a last know good value. Assuming those values arestored locally in the I/O module in step S404, the outputs can be set tothose values.

Ease of Configuration

There are several factors that contribute to a so-called “zeroconfiguration” system. One factor is the use of a point-to-point system.This avoids the problems of complicated networks and particularly iteliminates many configuration issues. Another is a simple method pairingof units to know each other's address. This can be done by programmingduring manufacturing and providing them in pre-paired units. It can alsobe accomplished by other methods in the field that are presented below.Since this system is modular with one controller supporting severalplug-in I/O modules, there is also a need to provide the controller witha mechanism to direct to and from each I/O module. In the currentlypresented embodiment this is done by rotary switches on the I/O modulesthat are set to unique values.

Some systems, like the embodiment presented, support a default,fail-safe output state for each output. To do this in a rich manner canbe accomplished by software settings. In this embodiment, these statesare set by mechanical switches on the I/O modules, avoiding softwaresetup.

Indications of fault can also be an area for configuration. One verysimple way to accomplish this with the presently described embodiment isto tie one input at the remote end to ground, or leave open if thesignal type permits. At the control-room end the signal will be normallycontinually low. However, if the “fail-safe” state of that output is setto high, failure or attack will force it to a high state by the normaloperation of the system. Heuristics can be used to attempt todistinguish tampering attempts from other conditions.

Variations

In versions of this embodiment the radio circuitry might be integratedwith the controller circuitry rather than being a modularized,replaceable unit. In versions of the embodiment the modules may notconform to DIN 15 mounting specifications. Versions might use adaisy-chained bus between modules rather than a passive backplane.

Pairing of units and assignment of master/slave roles can be done in thefield rather than by factory settings. Versions can completely free ofrequiring software settings or comprise both software settings and localphysical switch settings.

I/O modules can be intelligent rather than just reproducing signals at adistance. For example an I/O module could have circuitry for directconnection to specific sensors. Or an I/O module could include a PID. Inthe case of an intelligent output module the concept of fail-safe wouldbe more complicated but still constitute a valuable feature.

Alternate Embodiment

An alternate embodiment has the controller and input/output functionscommonly packaged rather than modularized. A variation on thisembodiment would have the radio separately packaged and cabled to themain unit.

These teachings may be susceptible to various modifications andalternative forms; specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural referents unless thecontext clearly dictates otherwise.

It is claimed:
 1. A wireless-to-wire subsystem comprising: (a) acontroller having a radio module port adapted to connect to a radiomodule, the controller comprising logic circuitry to receive a sequenceof data packets over the radio module port, the data packets furthercomprising at least one data field encoding a signal value; thecontroller comprising logic for detecting an interruption of thesequence of data packets and the controller further comprising asubsystem interconnect bus; (b) at least a first I/O module comprisingat least one physical switch and circuitry emitting at least oneelectrical output signal, the first I/O module adapted to communicatewith the controller via the subsystem interconnect bus wherecommunication comprises the first I/O module receiving at least aportion of the data fields of the sequence of the data packets from thecontroller; the first I/O module further comprising logic circuitryconfigured to reflect the encoded value in a first data field to a firstoutput signal, and to update the value of the first output signalrepeatedly as data packets are received; and further, the subsystemconfigured to force the first output signal to a default stateassociated with the first output signal upon detection of aninterruption of the sequence of data packets by the controller; thedefault state associated with the first output signal being determinedby the settings of the at least one physical switch, a first settingindicating a first predetermined electrical state, a second settingindicating a second predetermined electrical state, and a third settingdesignating the default state as the state that was encoded in the firstdata field of the last packet received prior to the interruption.
 2. Thewireless-to-wire subsystem of claim 1 where at least some data packetsfurther comprise a second data field and the at least one I/O modulefurther comprises circuitry emitting a second output signal associatedwith the second data field, the second output signal having a respectiveassociated default state independently determined by the at least oneswitch from that of the default state associated with the first outputsignal.
 3. The wireless-to-wire subsystem of claim 1 where the at leastone I/O module further comprises: (a) circuitry configured to receive atleast one electrical input, and (b) logic circuitry adapted to encodethe value of a first of the at least one electrical inputs into a datafield of an outgoing data packet and to present the outgoing data packetto the radio module port for transmission.
 4. The wireless-to-wiresubsystem of claim 1 where at least a portion of some of the datapackets comprise encrypted data.
 5. The wireless-to-wire subsystem ofclaim 1 where the controller has circuitry to control the power fed tothe radio module port and is configured to use that facility to powercycle an attached radio module in cases when an attached radio module isnot responsive to the controller.
 6. The wireless-to-wire subsystem ofclaim 1 where the first I/O module and the controller are packagedphysically distinct from each other and where each package has anexternal connector conveying the subsystem interconnect bus.
 7. Thewireless-to-wire subsystem of claim 6 further comprising a passivebackplane configured to operationally couple to the external connectors.8. The wireless-to-wire subsystem of claim 1 further comprising a radiomodule operatively coupled to the radio module port.
 9. Thewireless-to-wire subsystem of claim 8 where the controller is adapted toconfigure the radio module for point-to-point communication.
 10. Thewireless-to-Wire subsystem of claim 8 where the radio module is adaptedfor communication using an IEEE 802.15.4 protocol.
 11. A systemcomprising the wireless-to-wire subsystem of claim 1 and an operativelycompatible wire-to-wireless subsystem, the wire-to-wireless subsystemcomprising: circuitry configured to receive an electrical input signal;logic circuitry configured to encode the value of the electrical inputsignal and to include the encoded value in an outgoing data packet andto initiate the outgoing data packet to be transmitted to thewireless-to-wire subsystem.
 12. The system of claim 11 where thewire-to-wireless subsystem and wireless-to-wire subsystem communicateusing a point-to-point master/slave protocol with one system acting as amaster and the other system acting as a slave.
 13. The system of claim12 where the system operating as a master has at least two, userselectable, periodic repetition rates, with the user's selectionmediated by a physical switch located within at least one of thesubsystems.
 14. The system of claim 13 where the wire-to-wirelesssubsystem and the wireless-to-wire subsystem each, respectively,comprise: electrical signal receiving circuitry, electrical signalemitting circuitry and are each configured to transmit packetscontaining encodings of received signals to the other subsystem and areeach also configured to receive packets from the other subsystemcontaining encoded signals to be emitted; whereby bi-directional signalmirroring is provided.
 15. A method for causing an emitted electricalsignal from a first system, comprising physical switches, to follow thestate of an electrical signal received by a distinct second system, themethod comprising, the first system: (a) receiving a series of unicastdata packets transmitted by the second system via a radio receiver inthe first system, the unicast data packets containing at least a firstdata field representing the state of the electrical signal received bythe second system; (b) updating the state of the emitted electricalsignal to correspond with the state of the electrical signal representedin the first data field; and (c) detecting a break in the series of theunicast data packets, and when a break is detected: (d) forcing theemitted electrical signal to a default state; the default state beingdetermined by the settings of at least one of said physical switcheslocated in the first system where a first setting signifies a firstpredetermined state, a second setting signifies a second predeterminedstate, and a third setting signifies the default state to be the statethat was represented in the first data field of the last of the seriesof unicast data packets received prior to the break in the series ofdata packets.
 16. The method of claim 15 where at least a portion ofsome of the data packets are encrypted.
 17. The method of claim 15 whereat least some packets contain at least a second distinct data field andthe first system comprises an associated second distinct emitted signal,the updating and forcing steps being carried out independently withrespect to the second data field and the associated second emittedsignal, the second emitted signal having independent default stateswitch settings.
 18. The method of claim 15 where the detecting isaccomplished heuristically.
 19. The method of claim 15 where thedetecting is at least partially based on untimely arrival of receivedtransmissions.
 20. The method of claim 15 further comprising: signaling,externally the detection of an anomaly detected in receiving, thesignaling distinguishing suspected attack from suspected technicalfailure.